The well known "chimney effect" is the cause of considerable heat loss from heated buildings, particularly residences having a conventional wood-burning fireplace with a masonry or other similar chimney that is blocked off by a manually operable flue damper. When the fireplace is in use, the damper is of course open. A difficulty arises that when the fire dies out, the damper must remain open as long as there is any partial burning or smoking, which means that the damper may very easily be left in its open position long after there is no need for it to be open. Even when closed, the conventional chimney damper does not provide a particularly tight seal, and if a fire is started without first opening the damper, the dwelling space may be quickly filled with smoke before the damper may be opened.